GaN (gallium nitride) is one of Group III-V compound semiconductors and has a wurtzite-type crystal structure belonging to a hexagonal crystal system.
In recent years, a GaN substrate being a single crystal substrate formed of only a GaN crystal has attracted attention as a substrate for a nitride semiconductor device.
A nitride semiconductor is also called a “III-nitride based compound semiconductor”, “nitride-based Group III-V compound semiconductor”, “GaN-based semiconductor” or the like, and includes, besides GaN, a compound where gallium in GaN is partially or fully substituted with other Group 13 elements (B, Al, In, or the like) in the periodic table.
One particularly useful GaN substrate is a C-plane GaN substrate having a main surface parallel to or substantially parallel to the C-plane.
The C-plane GaN substrate has a gallium polar surface being a main surface on the [0001] side and a nitrogen polar surface being a main surface on the [000-1] side. The gallium polar surface is currently mainly used for formation of nitride semiconductor devices.
A GaN crystal grown by an HVPE (Hydride Vapor Phase Epitaxy) method is used for a C-plane GaN substrate commercially produced.
A seed used in the growth of a GaN crystal by an HVPE method is a GaN template having a c-axis-oriented GaN film grown on a single crystal substrate different in composition from GaN, such as a sapphire substrate or a GaAs substrate, by an MOVPE (Metal Organic Vapor Phase Epitaxy) method. A GaN crystal is grown on the GaN template in the c-axis direction.
Various procedures are proposed in order to reduce the dislocation density of a GaN crystal grown by an HVPE method.
In a typical example, a growth mask for allowing ELO (Epitaxial Lateral Overgrowth) to occur is provided on a main surface of a seed substrate (Patent Document 1). According to this method, when crystals growing in a lateral direction mutually coalesce, a dislocation loop is formed to result in the disappearance of dislocation.
JP-A-2006-66496 (Patent Document 2) discloses a method of generating a surface pit with an inversion domain (a domain where the polarity of a crystal is locally inverted) on a GaN crystal. While the growth direction of the GaN crystal is the [0001] direction (+c direction) in any regions other than the inversion domains, it is the [000-1] direction (−c direction) in the inversion domains. It is mentioned that dislocation defects aggregating towards the surface pits are confined in the inversion domains.
One index for measuring performances and reliability of semiconductor devices is a current leakage in reverse bias. A device with less leakage is more excellent in reliability. With respect to a Group III nitride semiconductor device in which the main part of a device structure is formed with a Group III nitride semiconductor crystal, it is mentioned that a main crystal defect causing a current leakage in reverse bias corresponds to a spiral dislocation (Non-Patent Literature 1 and Non-Patent Literature 2). The term. “spiral dislocation” here refers to a pure spiral dislocation, and does not include a mixed dislocation (containing a spiral component and an edge component).